1. Field of the Invention
The present invention relates to a lithographic projection apparatus.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a pattering device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from United States patents U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a pattering device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
The ever present demand in lithography to be able to image mask patterns with ever decreasing critical dimension (CD) necessitates increasing overlay accuracy (the accuracy with which two successive layers can be aligned with respect to each other). This drives a need for ever increasing alignment accuracy. Since the overlay error must be much smaller than the critical dimension and the alignment error is not the only contribution to overlay error, a critical dimension of 90 nm demands an alignment accuracy of 10 nm or less.
A known through-the-lens (TTL) alignment system uses linear phase gratings of 16 xcexcm pitch etched onto the substrate which are illuminated by laser light. The diffracted light is then imaged on a reference grating. By scanning the substrate underneath the alignment system and detecting the light passing through the reference grating as a function of stage position, the position of the substrate can be estimated with nanometer accuracy. However, the known TTL alignment system uses one wavelength of laser light and is subject to process dependent errors. Such errors occur when previously produced process layers form diffractive structures affecting the wavelengths used in the alignment system. An alignment system using one wavelength of light is strongly affected by such errors, introducing a second frequency reduces these errors somewhat by averaging, since the different wavelengths will not be affected in the same way, but does not eliminate them entirely. Such errors can also be caused by asymmetrically deformed alignment marks.
U.S. Pat. No. 5,371,570 discloses a through the lens alignment system using broadband radiation to illuminate alignment marks on the wafer. However, the alignment radiation is produced by a halogen lamp. The beam produced by such a lamp has a high xc3xa9tendue (solid angle subtended by the beam multiplied by the area of the cross-section of the beam) therefore it is difficult to obtain a high measurement light intensity at the alignment mark, resulting in a low signal to noise ratio (SNR).
WO 98/39689 discloses an off-axis alignment system that uses multiple wavelengths and higher diffraction orders to avoid errors resulting from asymmetry of the alignment mark caused by chemical-mechanical polishing. The image of the grating is imaged for each color on a different reference grating to obtain a measurement signal.
U.S. Pat. No. 5,559,601 discloses an alignment system that uses laser diodes, e.g. providing four wavelengths, to illuminate mask and wafer marks. The wafer is scanned relative to the mask and alignment information derived by Fourier analysis of the intensity of the return radiation as a function of wafer position.
It is an aspect of the present invention to provide an improved alignment system, in particular one which is less susceptible to process-dependent effects.
This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to provide a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table that holds a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate; and an off-axis alignment system including a radiation source constructed and arranged to illuminate a phase grating on a substrate held on the substrate table and an imaging system constructed and arranged to image diffracted light from the phase grating onto an image plane, wherein the imaging system images the phase grating onto one single image plane substantially correctly at at least two distinct wavelengths.
The use of an imaging system capable of imaging at least two wavelengths correctly onto a single image plane is advantageous in that it is more robust than single wavelength alignment. The use of multiple colors effectively averages out certain errors in the alignment signal due to asymmetric marks and the detection at one single imaging plane makes it unnecessary to mix the detection single of the two distinct wavelengths. Furthermore it diminishes the effect of thin film interference effects on signal strength. Both a true broadband spectrum and a set of discrete (laser) wavelengths may be used.
An advantage of periodic structures (gratings) over non-grating mark types is that effectively only a part of the total NA of the imaging system is used because the light is diffracted in very distinctively determined orders by the grating. By using non-grating mark types the mark image will be equally distributed over the total NA of the imaging system, and will be equally sensitive to aberrations in the total area of the pupil. The effective area of the pupil that is being used is also determined by the NA of the illumination system, however that is of relatively small influence.
The alignment system may comprise an illumination system for illuminating the phase grating with an NA greater than 0.01 preferably greater than 0.1 and most preferably greater than about 0.2. The use of an illumination NA larger than 0.01 is advantageously to get enough light on the grating, which is a problem for broadband sources having a high etendue. Laser sources commonly used for illuminating purposes have a low etendue and therefore there is no need for illuminating with a NA higher than 0.01 to get enough light upon the grating. Just the plane wave of the laser is radiated upon the grating.
Another advantage of illuminating with a relative high NA is that this makes the system less sensitive to illumination angle dependent errors. If the grating is illuminated from one direction, all the radiation from that one direction may suffer from the same illumination angle dependent error so that the total alignment signal is dependent on that error. In a higher NA illumination system the radiation is distributed over different illumination angles so that the illumination angle dependent errors are averaged out for the different angles.
A drawback of the relatively high illumination NA is that the grating must be in the focal plane of the illumination beam. The high illumination NA and the grating period make it further necessary that the imaging system for projecting diffracted light from the phase grating on the reference grating needs a relatively high NA. The imaging system may have a NA greater than about 0.7, preferably greater than 0.8, most preferably greater than about 0.9. The High NA of the imaging system makes also the imaging system focus sensitive. A separate focussing sensor is therefore needed in the alignment system.
Another drawback of the illumination system having a high NA is that the radiation of the illumination must have an homogeneous angular distribution. The use of a specially designed homogenizer may therefore be necessary.
The use of a small, e.g. less than 5 xcexcm, preferably 1 xcexcm, pitch grating enables a reduction of the interpolation needed in the data analysis/position estimation. This will proportionally decrease the influence of noise and mark asymmetry on the aligned position. Furthermore it enables the total area of the mark to be made as small as possible. Since the averaging of the alignment signal over the silicon area is actually related to the number of xe2x80x9cedgesxe2x80x9d that are present in the alignment mark, the averaging and thus insensitivity for local perturbations is increased by decreasing the period of the phase grating.
According to a further aspect of the invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; using a patterning device to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material; aligning the substrate to a reference grating by illuminating a phase grating provided on the substrate with radiation and imaging diffracted light from the phase grating onto the reference grating using an imaging system arranged to image the phase grating onto the reference grating substantially correctly at at least two distinct wavelengths.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.